This invention relates to a piston for internal combustion engines, particularly for supercharged diesel engines, comprising a piston head, which is formed with a combustion chamber recess provided with a heat-insulating layer of ceramic material having a low heat conductivity.
In internal combustion engines, particularly in diesel engines, the requirements for a lower fuel consumption and lower pollutant emission rates have resulted in an increase of the brake mean effective pressure performance (torque). That increase is achieved in many cases by the use of an exhaust gas turbocharger. The high engine output per unit of displacement results in such a high heat load on the piston that a more effective cooling of the piston is required from the aspects of strength and function. This is inconsistent with the fact that the dissipation of heat from the combustion process to the coolant and to the lubricating oil should be reduced, for the following reasons:
The quantity of lubricating oil, the quantity of coolant and the size of the radiator can be reduced. Less power is required for driving the fan. PA1 The fuel consumption can be reduced if a higher exhaust gas temperature is utilized with the turbocharged system. PA1 The quality of the exhaust gas can be improved, particularly when the engine is warming up, by an improved vaporization of the fuel which is distributed over the top surface of the piston head.
But the desired limitation or decrease of the heat to be dissipated by the coolant involves a high heat load on the piston head so that the latter must be heat-insulated.
Various proposals for the design of heat-insulated piston heads have already been made. J. H. Stang has investigated and has described in "Designing Adiabatic Engine Components", SEA 780,069 an aluminum piston provided with a screwed-on ceramic head, which is insulated from the base by steel discs. Temperatures up to about 900.degree. C. may be attained on the surface of the head of such piston. The problems involved in that "hot" piston reside in that a ceramic top which meets the strength requirements can be manufactured only at high costs and that the dead space above the first piston ring is relatively large.
In another aluminum piston, which is formed in its head with a combustion chamber recess, protected against the heat load by a ring which is embedded in the head casting at the rim of that recess and consists of a ceramic material having a low heat conductivity (.lambda.=2 W/mK), a low coefficient of linear expansion (.alpha.=1.5.times.10.sup.-6 /K) and a low modulus of elasticity (E=2.times.10.sup.4 N/mm.sup.2). That ring has an excellent compressive strength and consists of aluminum titanate (Al.sub.2 TiO.sub.5) and has a very high resistance to cyclic temperature changes.
The contraction of the aluminum material as it solidifies after the casting operation results in a shrunk joint between the ring and the surrounding aluminum material. The resulting compressive stresses ensure that the ring will be firmly held in position and give rise to compressive stresses in the ring itself. But such ceramic inserts can be provided only in such regions of the piston head in which the inserts are backed on a sufficiently large surface by the surrounding piston material.
From Laid-open German application No. 31 37 731 it is also known to provide the heads of pistons of internal combustion engines with a heat-insulating multilayer coating that is formed by a plurality of alternating layers applied by flame spraying and plasma spraying. Said layers consist of zirconium dioxide and/or zirconium silicate, metal and cerment and the outer layer consists of zirconium dioxide and/or zirconium silicate. Such sprayed-on layers must have a thickness of 1 to 2.5 mm if they are to have an adequate heat insulation value, and they will not adhere to the piston head unless the spray jet is directed as nearly as possible at right angles to the piston head surface which is to be coated and said surface has no substantial depressions or projections. These requirements are adequately met only in that region of the piston heat which is spaced around the combustion recess and at the top land whereas they are not reliably met in the combustion chamber recess and at the rim thereof.